The present invention relates to a pneumatic tire, more particularly to a sidewall structure capable of improving the durability of the tire under high speed running on dirt roads.
In pneumatic tires, when used on dirt roads at high speed, such as a rally, it is important to protect the tire sidewall from edged objects on the roads.
As a countermeasure therefore, as shown in FIG. 6, each sidewall portion (a) is provided with a plurality of axially protruding concentric ribs (b), wherein a part (c) between the adjacent ribs (b) is formed as constant-depth groove (hereinafter the xe2x80x9cgroove (c)xe2x80x9d). Such a tire is disclosed in Japanese Patent No. 2613996.
If the bottom of the groove (c) is cut during running, the cut tends to easily grow due to the presence of the ribs because tensile stress in the tire radial direction concentrates at the groove bottom when the tire is deformed.
Such a cut may be avoided if the amount of axial protrusion of the ribs is increased. But, an increase in the tire weight, which is especially undesirable for a racing tire, is unavoidable.
On the other hand, if the rubber thickness at the groove bottom is increased, namely, the depth of the groove (c) is decreased, part (c) may be increased in its resistance to cut. In this case, however, in addition to an increase in the tire weight, an unfavorable result is liable to be caused whereby cracks occur in the groove bottom because the tensile stress f which concentrates on the groove bottom increases as the groove depth decreases. Further, the shock absorbing performance of the tire is deteriorated.
It is therefore, an object of the present invention to provide a pneumatic tire suitable for high speed running on dirt roads, in which the resistance to cuts is improved and the occurrence of cracks is effectively controlled without increasing the tire weight.
According to the present invention, a pneumatic tire comprises a pair of sidewall portions each provided on the axially outer surface thereof with a side protector, the side protector comprising at least two concentric ribs and at least one circumferential groove defined therebetween, wherein the circumferential groove is composed of a series of deep parts and shallow parts which alternate in the tire circumferential direction.
The shallow parts connect the adjacent ribs to each other to resist the motion that opens the groove. Even if the deep parts are cut, as the tensile stress in the deep parts are effectively reduced by the presence of the shallow parts, the growth of the cut can be prevented. If the shallow parts are cut, the growth of the cut can be prevented because such parts have a sufficient amount of rubber thickness.